A Brief Introduction to Earthquakes

Stress… it can be a big deal.

We all get stress all the time — from any number of sources and in a dog’s breakfast of exotic flavors and intensities. Sometimes we bend and sometimes we shatter and break, and it seems to me that how we respond to stress can be determined as much by our physical state as it is by our mental and emotional status.

It works pretty much the same for the rocks of the earth’s crust—the reality of their physical condition in large measure controls what happens when they’re put under stress. The good news (or the bad) is that — thanks to plate motions — crustal stress happens all the time and in a great many locations around, and within, the globe. Throughout the fathomless reaches of earthtime our planet has remained an active and dynamic system, and any who might be interested — and even those who aren’t — will always have ample opportunity to observe first-hand how these stresses can affect the earth, and therefore all of us who live upon it’s surface.

And there’s no shortage of variables to muck up the issue: igneous rock vs. sedimentary vs. metamorphic; hard rock vs. soft; hot rock vs. cold… the infinite variations and combinations all contribute to how the crust will deform.

Of special and immediate interest to humankind are the hot… and the cold.

Volcanic eruptions are relatively benign insofar as their effect on humanity is concerned. This isn’t to imply that they can’t wreak havoc upon your day — rarely is the 1st Law of GeoFantasy more germane — but there is generally a clearly defined ramping-up period prior to any significant event. An individual, or community, will usually have time — if the warnings are understood and heeded — to get out of the way. (Click here for an earlier blog post that touches on this.)

But earthquakes are rude and give no timely warning at all. In response to the persistent creeping of the plates, the earth’s crust silently builds up pressure until it breaks, sending out waves of seismic energy in all directions — slowly bend a meter stick until it snaps and you’ll get the general idea. This build-up of elastic strain energy happens very slowly, however, and — in keeping with the 2nd Law of GeoFantasy — it may take many hundreds of years to reach the breaking point.

Since we don’t see or feel this part of the process, most of us tend to get lulled into the reasonable assumption that the ground beneath our feet is stable.

But it isn’t.

And we’re dealing with some determined and insatiable forces here: the plates continue to drift, stresses continue to build, and there can be only one possible outcome. It may take a bit of time, but sooner or later — depending on the physical characteristics of the material — the accumulating stress will finally exceed the strength of the rock and the rock will break.

Then we feel it!

But the only way a rock can break and release seismic energy is if it’s a solid (you can shatter ice, but not water), and that means that the material needs to be relatively cool. And that brings us to an imaginary region of change within the earth’s crust called the Brittle-Ductile Transition Zone.

You just gotta love the BDTZ. Fortunately, it’s way simpler that it sounds. Near the surface the earth’s crust is cold and brittle, and when you hit it with a hammer (or drift a slab of it into another of our planet’s crustal plates) it will fracture and break.

With depth, however, the energy increases. As it does, the rocks become progressively hotter and more ductile, until they finally turn into something approaching the liquid phase. But whether almost-liquid or plastic or some other non-solid physical state that we don’t understand, this material doesn’t break when you hit it with directed stress; it bends and folds.

The Brittle-Ductile Transition Zone is the transition zone (another clever name from the fine folks at GeoSpeak) between these two fundamentally different physical environments, and it seems to my simple way of thinking that it must be a mighty strange place. Sometimes it’s probably deeper and sometimes thicker or thinner or whatever, but wherever you are on the surface, if you dig far enough you’re gonna get there way before you get to the mantle.

All this leads us to an inescapable conclusion: since earthquakes involve the breaking of comparatively cold crustal rocks, quakes must only occur above the BDTZ and therefore relatively close to the surface.

The bad news is that since humanity lives on the surface, these sudden and devastatingly powerful lurches can affect us.

And they have and they do.

And they will.

Hang on tight!